Satellites & Moons
Sixteen satellites of Jupiter have so far been discovered. The four largest were discovered in 1610 by Galileo. They were subsequently named after mythological lovers of the god Jupiter (or Zeus in the Greek pantheon): Io, Europa, Ganymede, and Callisto. This tradition has been followed in the naming of the other moons. Modern observations have shown th
at
the mean densities of the largest moons follow the trend apparent in the solar
system itself. Io and Europa, close to Jupiter, are dense and rocky like the
inner planets. Ganymede and Callisto, at greater distances, are composed largely
of water ice and have lower densities. During the formation of both planets and
satellites, proximity to the central body (the Sun or Jupiter) evidently
prevented the more volatile substances from condensing.
Callisto is almost as big as
Mercury, and Ganymede is bigger than Mercury, being the largest satellite in the
solar system. If they orbited the Sun as independent bodies, they would be
considered planets. The icy crusts of these two bodies are marked by numerous
impact craters, the record of an early bombardment, probably by comet nuclei,
similar to the asteroidal battering that scarred the Earth’s Moon and other
inner solar-system bodies. Callisto is the least active of the Galilean moons
and has the most complete impact record, suggesting that its surface is the
oldest. In 1997 the Galileo probe discovered that it had a tenuous atmosphere of
hydrogen and carbon dioxide, and in 1998 the discovery of a variable magnetic
field on the moon suggested the possible presence
of an ocean of salty water beneath its crust, similar to that of Europa.
The Galileo probe detected a
magnetic field associated with Ganymede suggesting that it must have generated
enough internal heat to maintain a partially molten interior. It appears to have
a metallic core 400 to 1,280 km (250 to 800 mi) in diameter surrounded by a
mantle of ice and silicates, and a thick water-ice crust. The surface is a
mixture of two terrain types: 40 per cent is covered by highly cratered dark
regions that appear to be old, while the remaining 60 per cent is covered by
younger, light, grooved terrain probably formed by tensional fracturing or
release of water from below. The large craters on Ganymede have no relief,
probably due to slow and gradual adjustment into the soft icy surface; they are
called “palimpsests”. Ganymede has evidently had a complex geological history
Introduction Structure & Composition Important numbers Satellites & Moons
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